1. Field of the Invention
The present invention relates to the fabrication of integrated circuits. More particularly, the invention relates to a process and apparatus for depositing and etching dielectric layers on a substrate.
2. Background of the Invention
Semiconductor device geometries have dramatically decreased in size since such devices were first introduced several decades ago. Since then, integrated circuits have generally followed the two year/half-size rule (often called Moore""s Law), which means that the number of devices on a chip doubles every two years. Today""s fabrication plants are routinely producing devices having 0.35 xcexcm and even 0.18 xcexcm feature sizes, and tomorrow""s plants soon will be producing devices having even smaller geometries.
In order to further reduce the size of devices on integrated circuits, it has become necessary to use conductive materials having low resistivity and insulators having low dielectric constants (kxe2x89xa64.0) to reduce the capacitive coupling between adjacent metal lines. A conductive material of interest is copper which can be deposited in submicron features by electrochemical deposition. Dielectric materials of interest are silicon oxides that contain carbon. Combination of silicon oxide materials and copper has led to new deposition methods for preparing vertical and horizontal interconnects since copper is not easily etched to form metal lines. Such methods include damascene or dual damascene methods depositing vertical and horizontal interconnects wherein one or more dielectric materials are deposited and etched to form the vertical and horizontal interconnects that are filled with the conductive material.
Dielectric layers can be deposited, etched and filled with metal in multiple steps that typically require frequent transfers of substrates between processing chambers dedicated to specific steps. Preferred methods for depositing dielectric layers include two predominant dual damascene methods where lines/trenches are filled concurrently with vias/contacts. In a xe2x80x9ccounter-borexe2x80x9d scheme, a series of dielectric layers are deposited on a substrate as described in more detail for various embodiments of the present invention. Then vertical interconnects such as vias/contacts are etched through all of the layers and horizontal interconnects such as lines/trenches are etched through the top layers. In the alternative, the lines/trenches are etched in the top layers and then the vias/contacts are etched through the bottom layers. A conductive material is then deposited in both the vertical and horizontal interconnects.
The other predominate scheme for creating a dual damascene structure is known as a xe2x80x9cself-aligning contactxe2x80x9d (SAC) scheme. The SAC scheme is similar to the counter-bore scheme, except that an etch stop layer is deposited on a bottom dielectric layer and etched to define the vias/contacts before another dielectric layer is deposited on top of the etch stop layer. The vertical and horizontal interconnects are then etched in a single step, and conductive material is then deposited in both the vertical and horizontal interconnects.
The counter-bore scheme does not require an etch stop layer between the dielectric layers if the upper dielectric layer can be etched using conditions that provide an etch rate for the upper layer that is at least about three times greater than the corresponding etch rate for the lower layer (i.e., an etch selectivity of at least about 3:1). However, the selectivity of etch processes for conventional low k dielectric layers is typically less than 3:1, and etch stop layers that provide the desired etch selectivity are routinely used between adjacent low k dielectric layers. The etch stop layers provide uniformity in the depth of horizontal interconnects across the surface of the substrate. The etch stop layers further reduce micro-trenching such that the bottom of horizontal interconnects are flat instead of deeper at outside edges. The etch stop layers further reduce faceting or fencing of previously etched vertical interconnects during etching of horizontal interconnects, wherein the edge between the bottom of the horizontal interconnects and the side walls of the vertical interconnects are sharp instead of either rounded (i.e., faceted) or raised (i.e., fenced) depending on whether the side walls of the vertical interconnects are exposed to etch gases or shielded from etch gases.
Conventional etch stop layers provide the benefits just described for damascene applications, but typically have dielectric constants that are substantially greater than 4. For example, silicon nitride has a dielectric constant of about 7, and deposition of such an etch stop layer on a low k dielectric layer results in a substantially increased dielectric constant for the combined layers. It has also been discovered that silicon nitride may significantly increase the capacitive coupling between interconnect lines, even when an otherwise low k dielectric material is used as the primary insulator. This may lead to crosstalk and/or resistance-capacitance (RC) delay that degrades the overall performance of the device.
Ideally, low k dielectric layers would be identified and etch processes would be defined wherein an etch selectivity for the dielectric layers is at least about 3:1 for use in selective etch processes such as dual damascene processes. Preferably, the low k dielectric layers that provide the desired etch selectivity could be deposited in the same chamber.
The present invention provides a method for etching one or more dielectric layers having a dielectric constant less than or equal to about 4.0 (low k), wherein differences in dielectric compositions provides an etch selectivity of at least 3:1. The invention includes etching of dielectric layers containing silicon, oxygen, carbon, and hydrogen wherein differences in composition provide an etch selectivity greater than 3:1 in the absence of a carbon:oxygen gas, such as carbon monoxide. Addition of carbon:oxygen gases to one or more fluorocarbon gases provides fast etch rates at lower etch selectivity which can be used when high selectivity is not needed. At least one of the dielectric layers preferably has high carbon content (greater than about 1% by atomic weight) or high hydrogen content (greater than about 0.1% by atomic weight). The carbon:oxygen gas is reduced or omitted from etch gases when a selective etching of adjacent dielectric layers is desired, such as when forming horizontal interconnects.
The present invention further provides an integrated method for depositing and etching adjacent low k dielectric materials with reduced transfers of a substrate between chambers, and with an etch selectivity between adjacent dielectric layers of at least 3:1. The high etch selectivity provides horizontal interconnects having uniform depths and substantially square corners without conventional etch stop layers. At least one dielectric layer contains silicon, oxygen, carbon, and hydrogen. Additional dielectric layers can be any dielectric layer having a dielectric constant less than about 4.0, such as produced by spin on deposition methods or by chemical vapor deposition methods. All dielectric layers are preferably produced by chemical vapor deposition of one or more organosilicon compounds using power levels, flow rates, and composition changes to control etch selectivity by controlling the silicon, oxygen, carbon, and hydrogen content of the deposited materials.
In a first preferred dual damascene embodiment, a first low k dielectric layer and a second low k dielectric layer are deposited on a substrate by oxidation of one or more organosilicon compounds, such as methylsilane, CH3SiH3, or hexamethyldisiloxane, (CH3)3xe2x80x94Sixe2x80x94Oxe2x80x94Sixe2x80x94(CH3)3, for subsequent etching of vertical and horizontal interconnects. The first dielectric layer is an etch stop layer that contains silicon, oxygen, carbon, and hydrogen, preferably at least about 5% carbon by atomic weight and at least about 1% hydrogen by atomic weight. The second dielectric layer preferably contain less than two-thirds of the carbon or less than one-fifth of the hydrogen contained in the first dielectric layer, more preferably less than one-half of the carbon or less than one-tenth of the hydrogen. The vertical and horizontal interconnects are then etched into the low k dielectric layers using fluorocarbon gases. A carbon:oxygen compound, such as carbon monoxide, is added to the fluorocarbon gases during etching of vertical interconnects, and is not used during etching of horizontal interconnects to obtain an etch selectivity of at least 3:1. The horizontal and vertical interconnects can then be filled with a conductive material such as copper.
In a second preferred dual damascene embodiment, a first low k dielectric layer, a second low k dielectric layer, and a third low k dielectric layer are deposited on a substrate by oxidation of one or more organosilicon compounds, such as methylsilane, CH3SiH3, or trimethylsiloxane, (CH3)3xe2x80x94Sixe2x80x94Oxe2x80x94Sixe2x80x94(CH3)3, for subsequent etching of vertical and horizontal interconnects. The second dielectric layer is an etch stop layer and contains silicon, oxygen, carbon, and hydrogen, preferably at least about 5% carbon by atomic weight and at least about 1% hydrogen by atomic weight. The first and third dielectric layers preferably contain less than two-thirds of the carbon and less than one-fifth of the hydrogen contained in the second dielectric layer, more preferably less than one-half of the carbon and less than one-tenth of the hydrogen. The vertical and horizontal interconnects are then etched into the low k dielectric layers using fluorocarbon gases. A carbon:oxygen compound, such as carbon monoxide, is added to the fluorocarbon gases during etching of vertical interconnects, and preferably is not used during etching of horizontal interconnects to obtain an etch selectivity of at least 3:1. The horizontal and vertical interconnects can then be filled with a conductive material such as copper.